It is hard to diagnose a problem over the internet, but, here are some pointers:
When in growth water often and allow plants to remain moist. When not actively growing, water plants sparingly and allow to dry out between watering. Yellowing of new leaves indicates over watering. Yellowing of older leaves is natural and as the leaves are shed, new ones will take their places. Hope this information is helpful!It also helps to fertilize regularly during the growing period.